1. Field of the Invention
The present invention relates to an image formation method, electrophotographic toners for use in the image formation method, and a printed matter produced by the image formation method.
2. Discussion of Background
Conventionally, there are known a method and apparatus for full-color electrophotography for the formation of multi-color images, which can be attained by repeating an image formation process comprising the steps of forming latent electrostatic images on a latent electrostatic image bearing material such as an electrophotographic photoconductor, based on a color image information, developing the latent electrostatic images with toners with colors corresponding to the latent electrostatic images to form toner images, transferring the toner images to an image receiving material, and fixing the toner images to the image receiving material with the application of heat thereto, thereby obtaining multi-color images.
It is required such multi-color images have an appropriate glossiness, for instance, in the case of reproduction of multi-color images from photographs. It is also required that the toner layers of the obtained images be made flat. In order to obtain such flat toners layer, binder resins with a low melting point are generally used in the color toners.
In recent years, electrophotographic color copying machines and color printers are widely used. The reproduction of full-color images by such a color copying machine or a color printer is relatively good in quality. However, monochrome images are not reproduced often by the color copying machine or the color printer. This is because the speed of the reproduction of monochrome image by the color copying machine or the color printer is lower than that by a monochrome copying machine or a monochrome printer, and the monochrome image reproduced by the color copying machine or the color printer and the monochrome image reproduced by the monochrome copying machine or the monochrome printer differ in quality. Therefore, currently, color images are reproduced by the color copying machine or color printer, while monochrome images are exclusively reproduced by the monochrome copying machine or the monochrome printer. In other words, currently the color copying machines and color printers are clearly segregated from the monochrome copying machines and monochrome printers in terms of the application. This makes it difficult to expand the use of the color copying machine or color printer in offices where monochrome images are mainly reproduced.
In particular, with respect to the monochrome image, a mat finish, which is not shiny, is conventionally preferred. In other words, there is a tendency that a monochrome image produced by the color copying machine or the color printer, which is shiny, is not preferred.
With respect to a black toner, there is a method of controlling the glossiness of the image produced by the black toner by containing therein a polymeric resin with a relatively high melting point. The polymeric resin is the same polymeric resin as used in the black toner for use in the monochrome copying machine or the monochrome printer. For example, in Japanese Laid-Open Patent Application 6-148935, it is proposed to control the molecular weight distribution and the fused melting point of a resin component used in a black toner, and to change the quantity of heat applied to the black toner, depending upon the choice of monochrome copy image formation or color copy image formation, thereby controlling the glossiness of the image produced by the black toner. Even if the various properties of the black toner are controlled, it is still possible that in the multi-color reproduction, the glossiness of the color images produced by the color toners other than the black toner markedly differs from the glossiness of the black image produced by the black toner. When this takes place, for instance, in an image of a human face reproduced from a photograph which includes black portions, the reproduced image may look considerably awkward. In particular, when a character portion and a photographic portion are mixed in an image to be reproduced from a full-color photograph, a significant difference in glossiness is caused between a highlight portion in the photographic portion and the character portion when reproduced. This will make both the characters and other images look bad.
Generally, the glossiness of a toner image tends to be proportional to the amount of the toner transferred for the formation of the toner image. Therefore, an image developed with a small amount of a toner tends to have a low glossiness, while an image developed with a large amount of a toner tends to have a high glossiness. These characteristics can be advantageously utilized for controlling the contrast of monochrome toner images to some extent. However, in the reproduction of a full-color image, in particular, from a photograph, a larger amount of a black toner tends to be transferred to a black portion of the image since the black portion has a higher density in comparison with the other portions of the image.
In the case where each of the black toner and the color toners other than the black toner produces an image with the same glossiness when used alone, a full-color image produced from a full-color photograph by use of such color toners and black toner does not look good with an unbalanced glossiness of each color. This is because black portions in the full-color image tend to have an extremely higher glossiness in comparison with the other color portions.
In order to solve the above-mentioned problems, Japanese Laid-Open Patent Application 10-268562 proposes a color copying machine capable of providing a pre-determined difference in glossiness between a color portion produced by color toners and a black portion produced by a black toner in a color image to be reproduced. The proposed color copying machine, however, cannot completely control the glossiness of the image reproduced by the black toner to form a mat black image when a monochrome image is reproduced, so that this color copying machine cannot be used as a monochrome copying machine as well.
On the other hand, the adjustment of the glossiness of the image obtained depends not only upon the above discussed conditions for each toner, but also upon the amount of each toner used on the recording material, the structure of an image fixing unit used, and image fixing conditions adopted in the image formation method. For example, in Japanese Laid-Open Patent Application 4-1670, there is proposed an image fixing unit which is capable of selecting image fixing conditions in accordance with the kind of recording material employed. However, the black color in the color image cannot be adjusted only by changing the image fixing conditions described in the reference.
Color toners have problems that the coloring performance and the light transmittance through an OHP sheet are lowered due to imperfect fusing of the color toners. In order to prevent such problems, an oil is applied to the image fixing roller for the color copying machine, thereby carrying out sufficient image fixing and preventing the offset of the toners at image fixing. However, in many cases, the application of such an oil to the image fixing roller will have an adverse effect on the glossiness of the image reproduced.
It is therefore a first object of the present invention to provide an image formation method capable of controlling the glossiness of a black portion in a multi-color image in multi-color formation, thereby producing a well-balanced, good-looking image, and also capable of producing a mat monochrome image in monochrome image formation, which is preferred by the user.
A second object of the present invention is to provide a set of color toners including a black toner for use in the above-mentioned image formation method.
A third object of the present invention is to provide a printed matter produced by the above-mentioned image formation method.
The first object of the present invention can be achieved by an image formation method capable of forming an image, using an image formation apparatus capable of performing (1) multi-color image formation and printing by superimposing toner images with different colors by use of a plurality of color toners with different colors, including at least a black toner, and (2) monochrome image formation and printing, using only a black toner, with a maximum transferable amount of each color toner being in a range of 4xc3x9710xe2x88x923 kg/m2 to 8xc3x9710xe2x88x923 kg/m2, wherein an image glossiness (GKC) obtained by the black toner in the maximum transferable amount thereof at the multi-color image printing, an image glossiness (GCC) obtained by each of the color toners other than the black toner in the maximum transferable amount thereof at the multi-color image printing, and an image glossiness (GKM) obtained by the black toner in the maximum transferable amount thereof at the monochrome image printing satisfy the relationship of:
4xe2x89xa6GKCxe2x89xa625 (%),
xe2x80x835xe2x89xa6GCCxe2x89xa630 (%),
1xe2x89xa6GKMxe2x89xa610 (%),
0.5xe2x89xa6GKC/GCCxe2x89xa60.9,
0.1xe2x89xa6GKM/GKCxe2x89xa60.7, and
GKMxe2x89xa6GKCxe2x89xa6GCC.
In the above image formation method, the multi-color image printing and the monochrome image printing respectively comprise multi-color image fixing and monochrome image fixing which are conducted at an identical image fixing temperature and at an identical nip pressure, with the respective image fixing line speeds satisfying the conditions of:
1.2xe2x89xa6(image fixing line speed at monochrome image printing/image fixing line speed at multi-color image printing)xe2x89xa62.0.
Furthermore, in the above-mentioned image formation method, each of the plurality of the color toners other than the black toner may be an electrophotographic toner comprising a coloring agent, and a resin component with a weight-average molecular weight (Mw) of 10,000 to 25,000, and a number-average molecular weight (Mn) of 2,000 to 7,000, with the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn), Mw/Mn, being 3 to 10, and the black toner may be an electrophotographic toner comprising a coloring agent, the above-mentioned resin component, and at least one sub-resin component which is non-compatible with the resin component in an amount of 1 to 10 parts by weight to 100 parts by weight of the resin component.
The plurality of the color toners other than the black toner may comprise a yellow color toner, a magenta color toner, and a cyan color toner.
As the resin component, polyester resin can be employed. It is preferable that the sub-resin component have an MI value of 1 to 30. The MI value indicates the value of the melt index of the sub-resin component.
As the sub-resin component, styrene/acrylic resin can be employed.
Furthermore, in the above image formation method, the multi-color image fixing and the monochrome image fixing can be conducted, using an image fixing roller which is coated with a silicone oil component with a viscosity of 1 to 1000 stokes in an amount of 3xc3x9710xe2x88x925 kg/m2 to 8xc3x9710xe2x88x925 kg/m2.
The second object of the present invention can be achieved by a set of toners, comprising a plurality of color toners including a black toner, wherein each of the color toners other than the black toner is an electro-photographic toner comprising a coloring agent, and a resin component with a weight-average molecular weight (Mw) of 10,000 to 25,000, and a number-average molecular weight (Mn) of 2,000 to 7,000, with the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn), Mw/Mn, being 3 to 10, and the black toner is an electrophotographic toner comprising a coloring agent, the above-mentioned resin component, and at least one sub-resin component which is non-compatible with the resin component in an amount of 1 to 10 parts by weight to 100 parts by weight of the resin component.
In the above-mentioned set of toners, the plurality of the color toners other than the black toner may comprise a yellow color toner, a magenta color toner, and a cyan color toner.
As the resin component, polyester resin can be employed. It is preferable that the sub-resin component have an MI value of 1 to 30. As the sub-resin component, a styrene-acrylic resin can be employed.
The third object of the present invention can be achieved by a printed matter produced by the above-mentioned image formation method.